The fetal kidney

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With increasing sophistication of antenatal fetal ultrasound imaging, most anatomic abnormalities of the urinary tract are detected before delivery. An unfortunate consequence of this, however, has been an increasing number of children with transient anatomic variations which regress spontaneously and are of no consequence. The major such finding is mild dilatation of the fetal renal pelvis. Although a few fetuses with such a finding prove to have severe vesicoureteral reflux or ureteropelvic junction obstruction, most are eventually proven to be normal.

In an effort to distinguish the types of fetal renal pelvic dilatation which require careful post-natal follow up, a variety of calculations have been suggested. In the current issue of The Journal, Leung et al from Hong Kong propose the use of the “hydronephrosis index,” which divides the renal pelvic diameter by the volume of the fetal bladder. This makes intuitive sense because bladder dilatation is a common cause of renal pelvic dilatation. The authors have developed a nomogram for this measurement at varying fetal ages.

This measurement is likely to be operator-dependent, and these findings will need to be confirmed in other centers with larger numbers of pregnancies. Nonetheless, this line of inquiry may eventually help us to separate the occasional child with underlying serious urinary tract anomaly from the bulk of children who are normal. This will save resources and certainly avoid needless parental stress.